Recently, microwave-assisted magnetic recording (MAMR) has been used as a recording method for improving the surface recording density of magnetic media, such as magnetic disks used in magnetic disk drives. In MAMR, in addition to the magnetic field that emanates from the main pole, an alternating current (AC) magnetic field from a spin-torque oscillator (STO) is applied to a medium. High quality magnetic recording is facilitated due to the coercive force of the medium being lowered when the AC magnetic field is applied to the medium. Thus, in MAMR, it is important to develop an STO that generates a sufficiently large AC magnetic field in order to effect the lowering of the coercive force of the medium.
As shown in FIG. 5, according to the prior art, a STO 500 may be constructed from a field generation layer (FGL) 502 for generating an AC magnetic field, an interlayer (also referred to as a spacer) 504, and a spin polarization layer (SPL) 506 for transmitting the spin polarized torque. By conducting current 510 to the STO 500 when a magnetic field 508 is applied from a writer to the STO 500, the STO 500 oscillates, and an AC magnetic field is applied to the medium. In order to generate a large AC magnetic field, by effectively providing the spin torque, the STO 500 must oscillate in a state where the entire magnetization of the FGL 502 is directed in-plane.
A structure that directly laminates a perpendicular anisotropic film below or above the FGL 502 has also been used. The objective of this structure is to produce a single magnetic domain which emanates from the various FGL 502 magnetic domains.
However, an adequate assist effect cannot be obtained because the AC magnetic field that is generated in the STO 500 structures used currently is low. Thus, the magnitude of the spin torque hitting the FGL 502 is not sufficiently strong. One reason for this is that because the spin torque acts strongly at the boundary plane of the FGL 502 and the SPL 506, the FGL 502 magnetization at a position close to the SPL 506 is oriented in the in-plane direction of the film. However, the FGL 502 magnetization at a position far from the SPL 506 is oriented in the direction perpendicular to the film. Therefore, the problem in the development of the STO 500 capable of generating a high AC magnetic field is to orient all of the FGL magnetization in the film plane.